ИСТИНА

Yb‐doped Scheelite‐like molybdate and tungstate single crystals possess the efficient UV to NIR quantum cutting via cooperative energy down-conversion. They can be applicable for improvement the efficiency of crystalline silicon-based solar cells, due to the conversion of blue and UV sunlight photons into twice number of secondary photons with the energy just above the silicon band gap. The mechanism for this down-conversion involves some donor centers existing in the crystal. However, the nature of these donor centers in scheelite-like crystals remains unknown. The purpose of this research was to test two versions of that nature: Yb2+ ions, and the high-lying charge transfer excited states of Yb3+ ions. The Yb:CaWO4 crystal was selected for the research, because of the largest bandgap among all crystals of the scheelite family, 5.2 eV, as well as due to the most favorable crystallochemical conditions for Yb2+ formation. The series of Yb:CaWO4 and Yb,Nb:CaWO4 single crystals was grown by Czochralski in air, in N2, and in extra-pure argon atmospheres. Some samples were then additionally annealed in air, in a CO / CO2 atmosphere, and in a forevacuum. We studied the optical absorption spectra of the crystals, as well as the luminescence spectra under the excitation at different wavelengths of UV range. It has been determined that the additional introduction of Nb5+ into Yb:CaWO4 crystal raises the distribution coefficient of Yb3+ in the crystal by an order of magnitude, upto almost unity. Under the UV-excitation the Yb3+ downсonversion luminescence was observed at 1 µm. Increasing the oxidizing conditions of the samples fabrication and Nb5+ introduction into their composition reduce the intensity of this luminescence. Based on the data obtained, it was concluded that Yb2+ ions are the main donors involved into the down-conversion and the charge transfer excited states of Yb3+ ions are much less efficient donors. It was also confirmed that color centers and self-trapped excitons on tungstate complexes, do not participate in the down-conversion in Yb doped scheelite-like crystals.